Rib waveguides (also known as ridge waveguides) are known and used in optical transmission arrangements such as, for example, arrangements constructed upon and using silicon-on-insulator (SOI) devices. In an SOI arrangement, a rib waveguiding structure comprises a slab/strip waveguide region within the upper silicon layer of the SOI structure, with a silicon “rib” disposed along a portion of the surface of the upper silicon layer (there may or may not be a relatively thin dielectric layer situated between the silicon surface layer and the rib). The rib itself is surrounded by a material of lower refractive index, thus forming a cladding for the waveguide. The optical mode, once coupled into this structure, is then guided within the rib and the underlying portion of the upper silicon layer.
However, such rib or ridge waveguides formed in an SOI platform are known to suffer losses due to reflection, optical mode mismatch and unwanted Fabry-Perot resonances. The reflection loss arises as a result of the abrupt change in the effective index at the input and output of the rib waveguide. The optical mode mismatch loss is associated with the lack of perfect overlap of the optical mode between the silicon surface waveguiding structure and the rib waveguide structure. Moreover, reflective steps at the substantially parallel end faces of the rib waveguide create unwanted optical resonances (i.e., Fabry-Perot cavity) modifying the intensity of light at the output of the waveguide as a function of the wavelength of the propagating optical signal. For efficient and robust input and output coupling of the optical mode in the rib, a gradual change in the effective index, as well as some sort of adiabatic mode converter, is needed.